Process for making carbon fiber

ABSTRACT

Carbon fibers made by a process which involves heating a polyacrylonitrile fiber to a temperature below its carbonization temperature in an oxidizing atmosphere, carbonizing the fibers in a nonoxidizing atmosphere, such that combined heat and tension is applied to the fiber during at least one of the above steps, and then heating the carbon fibers in an inert atmosphere for at least one hour at from 1,300*-1,800* C., are improved by subjecting the carbon fibers to a post-heat treatment in an inert atmosphere at elevated temperatures of about 2,300*-3,000* C. for a very short time of 30-90 seconds.

Roberts, Jr.

[ Oct. 9, 1973 PROCESS FOR MAKING CARBON FIBER [75] Inventor: Benjamin W. Roberts, Jr.,

Schenectady, NY.

[73] Assignee: General Electric Company,

Schenectady, N.Y.

22 Filed: Apr. 21, 1971 211 App]. No.: 136,134

[56] References Cited UNITED STATES PATENTS 10/1970 Johnson 23/209.4 8/1968 Hough et al. 23/2093 X 3,607,063 9/1971 Douglas et al. 23/2093 3,533,743 10/1970 Prescott et al.... 23/209.1 3,539,295 10/1970 Ram 23/209.1 3,656,904 4/1972 Ram 23/209.1 3,635,675 1/1972 Ezekiel 23/2091 OTHER PUBLICATIONS Allen et al. Nature Vol. 224, Nov. 15, 1969 Pages 684-685.

Primary Examiner-Edward J. Meros Attorney-Paul A. Frank, Charles T. Watts, Gerhard K. Adam, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman [5 7 ABSTRACT Carbon fibers made by a process which involves heating a polyacrylonitrile fiber to a temperature below its carbonization temperature in an oxidizing atmosphere, carbonizing the fibers in a nonoxidizing atmosphere, such that combined heat and tension is applied to the fiber during at least one of the above steps, and then heating the carbon fibers in an inert atmosphere for at least one hour at from 1,3001,800 C., are improved by subjecting the carbon fibers to a post-heat treatment in an inert atmosphere at elevated temperatures of about 2,3003,000'C. for a very short time of 30-90 seconds.

2 Claims, 3 Drawing Figures PATENTED [JET 9 I975 SHEET 10? 2 0 2m fw W t n A Y w ma M Z 5 1 PROCESS FOR MAKING CARBON FIBER Recent advances in composite materials indicate that carbon fiber composites have considerable potential for structural and engineering uses such as, forexample, in aircraft engines and airframes. Especially important are the physical properties of the carbon fibers, e.g., high strength, high stiffness, and low weight. W. Watt, et al., The Engineer, May 27, 1966, p. 815, disclose the development of high modulus and highstrength carbon fibers. The article states that carbonfiber-reinforced plastics composite materials have been made which are much stiffer than any reinforcedplastics available and with a stiffness to weight ratio surpassing that of metals.-

The Watts, et al. processes are further described in two US. patents to Johnson, et al., which disclose producing carbon fibers by passing a synthetic carbonaceous fiber (polyacrylonitrile) through a number of heat treatments under carefully controlled conditions. The first patent, US. Pat. No. 3,412,062, discloses a treatment schedule whereby fibers of polyacrylonitrile are wound onto a carbon former under slight tension and heated 22 hours in air at 220 C. Thereafter the fibers are heated from 200l,000 C. over 24 hours in an atmosphere of hydrogen, then the fibers are heated sulting fibers have a tensile strength of up to 260x10 psi and a Youngs Modulus of 60X10 psi.

The second patent, 'U.S. Pat. No. 3,532,466, discloses a modification of the first process which involves initially heating fibers of polyacrylonitrile held under tension in an oxidizing atmosphere at from 200-250 C. for a time sufficient to permit complete permeation of oxygen through the fibers, next carbonizing at a carbonizing temperature of to l,000 C. in a hydrogen atmosphere, and then producing fibers of maximum strength by heating the fibers in an inert atmosphere such as argon for at least 1 hour at from l,300 l,800 C. The product has a tensile strength of 320X10 psi and a Youngs Modulus of 35 l0 psi.

These two processes are further discussed by DH. Logsdail, Applied Polymer Symposia, No. 9, 245 (1969) which discloses that the first type (heat treated to 2,700 C.) has a high modulus of 60x10 psi and a moderate strength of 300x10 psi, while the second type (heat treated to only 1,500 C.) has a moderate modulus of 40x10 psi, but high strength of 450x psi. Carbon fibers of these types are commercially available as follows:

Young's Tensile Modulus Strength Material psi X 10 psi X 10 Morganite High Strength Fiber Type I (high modulus) 55-65 200-300 Type II (high strength) 35-45 350-450 Quite surprisingly, I have discovered a third type of carbon fiber which has both a high-tensile strength substantially equivalent to Morganite type II (hereinafter designated as carbon fiber Type II) and a high Youngs modulus substantially equivalent to Morganite Type I (hereinafter designated as carbon fibers Type I). Now my novel carbon fibers which I shall designate as carbon fibers Type III are prepared by subjecting the carbon fiber Type II to a post-heat treatment at a high temperature for a very short period of time. The novel Type Ill carbon fibers have a tensile strength of at least 350x10 psi and a Young's modulus of greater than 45X 10 psi.

In accordance with the present invention, I have discovered a method of making a high-tensile strength, high-Youngs modulus carbon fiber comprising initially forming a carbon fiber by heating a polyacrylonitrile fiber to a temperature below its carbonizing temperature in an oxidizing atmosphere, carbonizing the fiber at a carbonizing temperature in a nonoxidizing atmosphere combined heat and tension being applied to the fiber during at least one of the above steps, then heating the carbon fibers in an inert atmosphere for at least 1 hour at from 1,300-l ,800 C. and subsequently subjecting the carbon fibers to a post-heat treatment in an inert atmosphere at a temperature of 2,300-3,000 C. for 30-90 seconds. The carbon fiber produced has a tensile strength of at least 350X10 psi and a Youngs modulus of greater than 45x10 psi.

This invention will be more clearly understood from the following description taken in conjunction with the following drawing in which:

FIG. 1 is a photographic representation of a longitudinal section of a carbon fiber prior to the posttreatment having a high-tensile strength and a moderate-Youngs modulus.

FIG. 2 is a photographic representation of a longitudinal section of a carbon fiber after the post-heat treatment having a high-tensile strength and a high-Youngs modulus.

FIG. 3is a graphic representation of the effect on the properties of the fiber during the post-heat treatment withtime and at a constant temperature of 2,950 C.

The process of making high-tensile strength and high Youngs modulus involves a series of steps to convert the organic polymer fiber to a turbostratic, randomly stacked graphitic material. Initially polyacrylonitrile fibers (included also are some copolymers of acrylonitrile) are heated while under tension in an oxidizing atmosphere at from 200250 C. for a sufficie nt time to permit substantially complete permeation of oxygen throughout the individual fibers. Then the organic fibers are heated to a carbonizing temperature of up to about l,O00 C. under nonoxidizing conditions whereby the fiber shrinks and orientation of fiber particles occurs along the longitudinal axis of the fiber. Thereafter the carbon fibers are heated in an inert atmosphere, e.g., argon at a temperature of l,300-l,800 C. for at least 1 hour. These fibers have substantially the tensile strength and the Youngs modulus as discussed for the commercially available Type II carbon fiber.

It is now that I have found that the Youngs modulus can be substantially increased without decreasing-the tensile strength by a post-heat treatment. This treatment'involves subjecting the carbon fibers to an inert atmosphere, e.g., argon, at an elevated temperature of about 2,300-3,000 C. for a very short period of time. The preferred temperature is from 2,600-3,000 C. While there does appear to be a relationship between time and temperature, such that, the higher the temperature the shorter the time interval required, I have found that the time appears to be a critical factor and should be in the range of 30-90 seconds. When the time is too short there is a sharp decrease in the tensile strength with some increase in the Youngs modulus, whereas when the time period is too long the tensile strength again falls off sharply while the Youngs modulus remains in the range of the Type I carbon fibers.

Referring now to the drawing, FIG. 1 is a phasecontrast electron micrograph of a polyacrylonitrile fiber which has been heat treated to about 1,500 C. and as it exists prior to the post-heat treatment of my invention. Circled are two smalled nuclei consisting of four to eight randomly stacked carbon atom planes of 3.4 A. spacing. The nuclei have a distinct preferred orientation introduced in an earlier stretching operation at an elevated temperature. The graphite basal planes tend to be aligned with normals perpendicular to the fiber axis but with a range of orientation of order :30.

In FIG. 2, the carbon fibers are shown in a phasecontrast electron micrograph after a post-heat treatment at a temperature of 2,950 C. in a vertical carbon tube furnace heated by induction in gently flowing high purity argon. The residence time in the furnace was 50-70 secondsand no load was applied to the fiber bundles. it is observed that the short term heat treatment produces considerable alignment, growth and coalescence of the plane packets to a highly organized state. Bending of the turbostratic lattice and absence of transverse grain boundaries with respect to the fiber axis is noted. After the heat treatment at 2950" C., the

number of planes in a packet approaches 50. The

raised temperatures and coalescing into the large packets shown in FIG. 2. The high tensile strength of the original Type ll carbon fibers has persisted throughout the heat treatment and the Youngs modulus has progressively increased from 34x10 psi to 59Xl0 psi.

My invention is further illustrated by the following example; I

Samples of carbon fibers Type II (Morganite Type II) about 10 inches in length were passed through a vertical graphatization furnace. The furnace temperature was 2,950 C. i 50 C. and flowing argon was used to prevent oxidation of the carbon fibers. The furnace residence times and the properties of the carbon fibers are set forth in the table below.

Residence Av. Tensile Av. Youngs Time Strength( lOpsi) Modulus( IOpsi) Untreated 400 29 I5 sec. 21 I 43.5 30 sec. 264 46 60 sec. 358 49 72 sec. sec.

It is noted that the tensile strength is dependent on both the time and the temperature and, in some instances the optimum tensile strength may require more than 30 sec.

The results are graphically shown in FIG. 3. These clearly indicate that a post-heat treatment as herein described produces high tensile strength and high Youngs modulus carbon fibers. The heating time is a critical feature and while the Youngs modulus of the carbon fiber rises rapidly during the post-heat treatment, the tensile strength falls off during the initial phase of the heating and then rises again to a maximum as shown in FIG. 3.

It will be appreciated that the invention is not limited to the specific details shown in the examples and illustrations and that various modifications may be made within the ordinary skill in the art without departing from the spirit and scope of the invention.

I claim:

1. A process for making a carbon fiber comprising heating a polyacrylonitrile fiber to a temperature below its carbonizing temperature in an oxidizing atmosphere, carbonizing the fiber at a carbonizing temperature in a nonoxidizing atmosphere, combined heat and tension being applied to the fiber during at least one of the above steps, heating the carbon fiber in an inert atmosphere for at least one hour at from l,300-l ,800 C., such that the resulting carbon fiber has an initial tensile strength of about 400x10 psi and Youngs modulus of about 29x10 psi, and subsequently subjecting the carbon fiber to a post-heat treatment in an inert atmosphere at a temperature of about 2,950 C. for a short period of time as defined by FIG. 3 such that the carbon fiber has a tensile strength of at least 350 l0 psi and a Youngs modulus of greater than 45x10.

2. The process of claim 1, wherein the post-heat treatment of the carbon fiber produces alignment, growth and coalescence of the plane packets to a highly organized state as shown in FIG. 2, whereby the Youngs modulus is increased to greater than 45x10 

2. The process of claim 1, wherein the post-heat treatment of the carbon fiber produces alignment, growth and coalescence of the plane packets to a highly organized state as shown in FIG. 2, whereby the Young''s modulus is increased to greater than 45 X 106 psi. 